1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to a method for tuning the gyro sensor coil to eliminate non-reciprocity error sensitivities.
2. Description of the Prior Art
A fiber optic gyroscope comprises the following main components: (1) a light source, (2) a beamsplitter (either a fiber optic directional coupler or an integrated-optics Y-junction), (3) a fiber optic coil, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the beamsplitter into copropagating and counterpropagating waves travelling in the sensing coil. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias or error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. Such factors are both time-varying and unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
While appropriate coil winding techniques can minimize some of the bias errors found in the output of a fiber optic gyro, in practice it is not possible to attain a "perfect" winding pattern. Bumps, wrinkles and various errors will invariably occur in winding. To the extent that such asymmetries exist, winding patterns are rarely as intended and the reduction of output bias is inherently limited. That is, while an intended symmetric coil winding pattern may locate similarly situated fiber segments equal lengths from the fiber center, in practice the theoretical cancellation of Shupe error due to temperature change is not observed. Rather, the presence of such difficult-to-avoid winding errors as "drop outs" and the like regularly frustrate the fabrication of a perfectly symmetric sensor coil. Such winding errors are to be expected in view of the quite substantial task involved in the precise winding of a coil of, for example, one thousand meters of optical fiber. As a result of such unavoidable winding imperfections, time-dependent changes in fiber properties, bias effects, sometimes significant, appear in the outputs of even symmetrically-wound sensor coils.